1. Field of the Invention
The present invention relates to a switch for use in an optical communication network using wavelength division multiplexing.
2. Description of the Prior Art
The present invention is in the field of optical switches or optical switching nodes having a “multigranularity” architecture. The “granularity” concept relates to predefined sets of transmission resources (typically carrier wavelengths or wavelength division multiplexes). The resources of this kind of set can be considered as a whole for the purposes of some common processing (typically switching). A multigranularity architecture therefore takes account of different levels of granularity to switch the total traffic at a switch. For example, a portion of the total traffic can be switched at the “fiber” level, i.e. grouping together all wavelengths that can be conveyed by an optical fiber, which therefore corresponds to the highest level of granularity. Another portion can be switched at the band of wavelengths level, which corresponds to an intermediate level of granularity. A final portion can be switched at the wavelength level, which corresponds to the lowest level of granularity. Intermediate levels of granularity can be further defined.
Using a multigranularity architecture limits the increase in the complexity of the switches in optical networks.
Telecommunications are currently expanding at a very great rate, reflected in increasing demands for data transmission. Fiber optic transmission is particularly affected by this phenomenon and the quantity of data transmitted via optical networks is constantly increasing. This is reflected in an increase in the number of fibers installed in networks and the number of carrier wavelengths used.
As of now, an optical fiber is capable of transmitting up to 256 wavelengths and each wavelength can convey a data bit rate of 10 gigabits per second (1 Gbit=109 bits). Accordingly, depending on the number of fibers arriving at the input of the optical switch, the total bit rate to be switched can be in excess of several tens of terabits per second (1 Tbit=1012 bits).
An optical switch with a multigranularity architecture processes data bit rates of this magnitude by switching partly wavelengths and partly bands of wavelengths, i.e. single-wavelength channels and wavelength multiplexes, respectively. The switch can further process groups of bands. Another possibility would be to process only bands of wavelengths and groups of bands. To simplify the description, the remainder of the description covers, by way of example only, three levels of granularity: wavelength, band and “fiber”, the latter level corresponding to a special case of groups of bands combining all wavelengths that can be conveyed by an optical fiber.
FIG. 1 is a diagram of a prior art optical switching node with a multigranularity architecture.
With the multigranularity architecture, it has been possible to evolve from monoblock switching nodes to switching nodes consisting of a stack of subnodes. Each switching subnode is assigned to a corresponding level of granularity. Thus in the example shown there is a switching subnode FXC associated with the “fiber” level of granularity (which is a special case of groups of bands), a switching subnode BXC associated with the “band” level of granularity, and a switching subnode WXC associated with the “wavelength” level of granularity.
In FIG. 1, the incoming fibers IF are first routed to the input ports IP of the switching subnode FXC. A few of the incoming fibers IF are switched directly to the output fibers OF via the output ports OP of the switching subnode FXC. A fiber AF coming from the client is directly inserted at a fiber insertion port Pins of the switching subnode FXC. A fiber DF to the client is extracted from a fiber extraction port Pext of the subnode FXC. The fiber DF must be wavelength division demultiplexed for the client, but the demultiplexers are not shown in the figure. Fibers Fbf are inserted from the switching subnode BXC to the fiber insertion ports Pins of the subnode FXC. These fibers Fbf come from the band to fiber multiplexer Mux B→F which multiplexes the bands coming from the output ports OP of the switching subnode BXC. Finally, fibers Ffb are extracted from the subnode FXC via extraction ports and are sent to the input ports IP of the subnode BXC after the fibers are demultiplexed into bands in the fiber to band demultiplexer Demux F→B.
The same switching process is used at the next lower level of granularity, i.e. in the switching subnode BXC at the band level of granularity, as well as at the lowest level of granularity, i.e. in the switching subnode WXC at the wavelength level of granularity.
A few of the bands arriving at the input ports IP of the subnode BXC are switched to the output ports OP of the subnode BXC. A band AB coming from the client is directly inserted at an insertion port of the subnode BXC. A band DB sent to the client is extracted via an extraction port Pext of the subnode BXC. The bond DB must be wavelength division demultiplexed for the client but the demultiplexers are not shown in the figure. Bands Bλb are inserted from the switching subnode BXC at the insertion ports Pins of the subnode BXC. These bands Bλb come from the multiplexer Mux λ→B which multiplexes wavelengths from the output ports OP of the switching subnode BXC into bands. Finally, bands Bbλ are extracted from the subnode BXC via extraction ports and are sent to the input ports IP of the subnode WXC after the bands are demultiplexed into wavelengths in the band to wavelength demultiplexer Demux B→λ.
The same switching process is used again in the subnode WXC. A few of the wavelengths arriving at the input ports IP of the subnode WXC are switched to the output ports OP of the subnode WXC. Wavelengths Aλ coming from the client are directly inserted at insertion ports Pins of the subnode WXC. Wavelengths Dλ sent to the client are extracted via extraction ports of the subnode WXC.
This prior art architecture just described with reference to FIG. 1 uses separate switching matrices for each level of granularity (typically based on “crossbar” optical switches). The fiber level of granularity is processed in the switching matrix FXC, the band level of granularity is processed in the switching matrix BXC, and the wavelength level of granularity is processed in the switching matrix WXC. There is therefore a dedicated switching matrix for each granularity. For given numbers of input ports assigned to the three levels of granularity, this solution represents the optimum in terms of limiting the complexity and size of the overall system.
However, because the number of input/output ports of each switching matrix allocated to each level of granularity is fixed, this becomes a drawback if evolving the architecture to adapt it to changes in traffic with time is envisaged.
Consider a concrete example of this kind of architecture with a bit rate of 10 Gbit/s per wavelength, 16 wavelengths per band and 10 bands per fiber. It may be necessary to switch:
in an initial step: 500 wavelengths, no band, no fiber, which represents a total bit rate of 5 Tbit/s;
in a second step: 250 wavelengths, 250 bands, no fiber, which represents a total bit rate of 42.5 Tbit/s;
in a third step: 100 wavelengths, 400 bands, no fiber, which represents a total bit rate of 65 Tbit/s;
in a fourth step: 100 wavelengths, 300 bands, 100 fibers, which represents a total bit rate of 209 Tbit/s; and
in a fifth step: no wavelength, 200 bands, 300 fibers, which represents a total bit rate of 512 Tbit/s.
The first step requires a 500×500 switching matrix WXC (which means a number of states of the matrix equal to 500×500) for the wavelength granularity. However, the switching matrix WXC will not be used completely in subsequent steps.
The third step requires a 400×400 switching matrix BXC for the band granularity. However, only half of the input/output ports of that matrix will be used in the fifth step.
Finally, the fifth step requires a 300×300 switching matrix FXC for the fiber granularity. Once again, the switching matrix is under-used in the other steps.
Accordingly, with the preceding example of evolution, using the prior art architecture, the total number of input ports to be provided in the optical switch is equal to 1 200, and those ports will be only partially used.
Also, the object of the present invention is to provide an architecture for switching different levels of granularity that avoids the drawbacks of the prior art, i.e. an architecture that is optimized not at a given stage of the evolution of the traffic to be switched but for a set of configurations adapted throughout that evolution.
To this end, the invention proposes to use only one switching matrix to switch all levels of granularity at the same time. The three separate switching matrices of the prior art, respectively corresponding to the fiber, band and wavelength levels of granularity, are replaced by a single switching matrix that processes all granularities. Depending on what is required, i.e. depending on the traffic to be switched, appropriate numbers of ports of the single matrix are respectively assigned to a low level of granularity (wavelengths), to an intermediate level of granularity (bands of wavelengths), and finally to a high level of granularity (fibers).